The present invention is directed to a semiconductor electromagnetic detector and a method of detecting electromagnetic radiation, and is directed particularly to a semiconductor infrared detector having doped quantum wells separated by finite-period superlattices.
Detection of electromagnetic radiation in the medium wavelength infrared (MWIR) band (wavelengths from 3-5 micrometers (.mu.m)) can be accomplished with several materials, e.g., indium antimonide (InSb). On the other hand, there is only one common detector for the long wavelength infrared (LWIR) band (wavelengths from 8-12 .mu.m): the narrow-band-gap semiconductor mercury cadmium telluride (HgCdTe).
Many problems with respect to the uniformity of optical and electrical properties and mechanical stability are associated with HgCdTe. As a result, routine fabrication of large HgCdTe detector arrays is not possible. In addition, detectors that are sensitive to more than one band of wavelengths at the same time (sometimes called "two-color" detectors) are not readily available.
Recently, semiconductor structures that have dimensions small enough to make quantum mechanical effects important have been described in the literature, e.g., M. Sundaram et al., "New Quantum Structures", Science vol. 254, pp. 1326-1335 (Nov. 29, 1991). Conventional quantum well infrared photodetectors (QWIPs) respond to thermal radiation with an increase in electrical conductivity that results from internal photoemission of charged carriers from energy states confined in quantum wells. Typical materials used for conventional QWIPs are III-V compound semiconductors such as gallium arsenide and aluminum gallium arsenide. A typical QWIP is a stack of layers in which the layers' materials and widths are carefully selected to achieve a desired distribution of energy states for the device's electrons. See, e.g., B. Levine et al., "New 10 .mu.m Infrared Detector Using Intersubband Absorption in Resonant Tunneling GaAlAs Superlattices", Appl. Phys. Lett. vol. 50, pp. 1092-1094 (Apr. 20, 1987).
An example of the energy states in a conventional, unbiased QWIP such as that described in the Levine et al. paper is illustrated in FIG. 1(a). Such a conventional QWIP is described in. General aspects of superlattices, which typically are structures of thin alternating layers of two materials having different electronic properties, are described in G. Dohler, "Solid-State Superlattices", Scientific American vol. 249, pp. 144-151 (November 1983). It will be understood that superlattices, as well as the other structures described in this application, can be fabricated by a wide variety of methods, e.g., vapor deposition methods such as molecular-beam epitaxy and electrochemical deposition methods. See, e.g., J. Switzer et al., "Electrodeposited Ceramic Superlattices", Science vol. 247, pp. 444-445 (Jan. 26, 1990).
Three of the QWIP's quantum wells QW are seen in FIG. 1(a), and the wells have widths and energy depths that are chosen to provide two confirmed states, the ground state and the excited state. The energy separation between the ground state and the excited state is set equal to the energy of the photon to be detected by the QWIP, and generally increases as the width of the layers corresponding to the quantum wells QW decreases. The well layers are doped with electron donor impurities, e.g., silicon, thereby partially filling the lowest energy state with electrons (as indicated in the figure by the dashed lines). Barriers having thicknesses of typically one hundred Angstroms (100 .ANG.) separate the quantum wells QW. In a QWIP consisting of a stack of GaAs wells and Al.sub.x Ga.sub.1-x As barriers, the depth of the wells can be varied over a range of about 500 mili-electron-volts (meV). As noted in the above-cited Sundaram et al. paper, the well depth is proportional to the difference between the bandgaps of the barrier and well layers, and varies almost linearly for 0.0&lt;.times.&lt;0.45.
When a bias is applied to the QWIP as illustrated in FIG. 1(b), the shape of the distribution of energy states changes. Electrons can jump into the excited states upon absorption of photons having the proper energy, and can tunnel through the barriers separating the wells; these electrons are collected as a current. Conventional QWIPs operate at biases corresponding to internal electric fields of 10-30 kV/cm, which are required for efficient extraction of the photogenerated carriers before recombination. Due to the electrons' tunneling from the ground state through the thin barriers, however, the current that flows in the absence of photons (the "dark current") is unacceptably large in the conventional QWIP.
Another type of QWIP is described in B. Levine et al., "High-Detectivity D*=1.0.times.10.sup.10 cm.sqroot.Hz/W GaAs/AlGaAs Multiquantum Well .lambda.=8.3 .mu.m Infrared Detector", Appl. Phys. Lett. vol. 53, pp. 296-298 (Jul. 25, 1988). An example of the energy states in such an unbiased QWIP is illustrated in FIG. 2(a). In this QWIP, the widths of the quantum wells QW are chosen so that only the ground state is bound in each well and the excited state is slightly above the barrier in the continuum. In such a QWIP, the barriers between the quantum wells are fairly thick, e.g., approximately 500 .ANG., thereby yielding lower dark current when a bias is applied as illustrated in FIG. 2(b). The carriers can still be collected because photon absorption raises the electrons into the continuum where they can be swept to the QWIP's contacts by the applied electric field before recombination occurs.
Although the QWIP illustrated in FIGS. 2(a)-2(b) has lower dark current than the QWIP illustrated in FIGS. 1(a)-1(b), it has only one bound state in each quantum well. Also, the energies of the first (virtual) excited states must be close to the tops of the barriers because the photon absorption strength (i.e., the probability of photoexcited transitions between states) decreases rapidly as the excited states move further above the barrier. For a given operating wavelength, these requirements can be satisfied by only one combination of well width and barrier height, eliminating any possibility of selecting detector performance by choosing different well widths and barrier compositions.
In addition, the carriers' effective mass must be low if only one bound state is allowed, thereby limiting such a QWIP to n-type dopants; this is disadvantageous because p-type dopants can lead to devices having lower dark currents. Furthermore, the conventional QWIP's spectral bandwidth can be changed over only a very small range due to fundamental properties of optical excitation of confined carriers to continuum states. As a result of these properties, changing the spectral bandwidth requires pushing the excited states deeper into the continuum, but as described above this dramatically reduces the photon absorption strength, or quantum efficiency.
A QWIP theoretically analyzed in D. Coon et al., "Narrow Band Infrared Detection in Multiquantum Well Structures", Appl. Phys. Lett. vol. 47, pp. 289-291 (Aug. 1, 1985) has a single quantum well and a barrier consisting of a superlattice. Aspects of the Coon et al. paper are described below, but it is important to note the paper admits that practical multiple-quantum-well detectors are beyond the scope of the model it describes. The superlattice barrier thicknesses and compositions described in the Coon et al. paper (superlattice periods, i.e., the distances between same-material layers, ranging from 114-94 .ANG.) provide a very narrow miniband of states (widths ranging from 6-18 meV, respectively). In addition, the paper describes thicknesses and compositions that position the first excited state of the quantum well in the lowest energy miniband of the superlattice in order to obtain a narrow bandwidth infrared detector that would have low dark current.
The Coon et al. paper suggests that electron transport normal to the plane of the quantum well should occur fairly easily. Nevertheless as described above, a bias is necessary to transport the photoexcited carriers for collection before recombination occurs. Applying even the lowest typical bias to a detector such as that described in the Coon et al. paper would render extraction of the photogenerated carriers extremely difficult and inefficient.
As described in E. Mendez et al., "Stark Localization in GaAs-GaAlAs Superlattices under an Electric Field", Phys. Rev. Lett. vol. 60, pp. 2426-2429 (Jun. 6, 1988) and R. Leavitt et al., "Stark Ladders in Strongly Coupled Superlattices and Their Interactions with Embedded Quantum Wells", Phys. Rev. B vol. 41, pp. 5174-5177 (Mar. 15, 1990), for example, miniband states become strongly localized when the potential drop across one superlattice period is comparable to the width of the miniband. The condition for localization of the miniband states to a single superlattice well is the potential drop across one period should be equal to one-half of the miniband width. Even the lowest bias typically necessary to transport carriers out of a QWIP before recombination would completely localize the miniband states of the superlattices in the detector described in the Coon et al. paper.
As a result of such localization, transport of photoexcited carriers in the detector described in the Coon et al. paper could occur only by tunneling between the localized miniband and quantum well states. It will be appreciated that a tunneling transport process cannot be characterized as easy. Moreover, although the paper suggests that remarkably high quantum efficiencies might be achieved, in fact a detector such as that described in the paper would have a very low quantum efficiency because the probability of the carriers' transport through the detector by tunneling is significantly less than the probability of their recombination.
In addition, the Coon et al. paper describes a theoretical treatment that is valid only for miniband widths that are much smaller than the depths of the superlattice potential wells. Thus, much of the latter portion of the paper and even its title are directed to considerations arising out of the narrowband limitation. Such a narrow miniband would result in a detector having an advantageously low dark current, but would introduce other problems. The minibands described in the paper are so narrow that, as the paper acknowledges, it would be a challenge even to position the quantum wells' excited states in resonance with the minibands.
The paper's approximate solutions of Schroedinger's equation are suitable for detectors having minibands that are narrow to reduce dark current, but do not permit accurate performance prediction, i.e., selection of predetermined properties, of biased MBT detectors having wide minibands. In fact, the paper inaccurately suggests that the QWIP's absorption spectrum would have the same width as the miniband and that the absorption strength would be almost constant throughout this width. Thus, it is not surprising that the paper is silent on selectively positioning the excited states within the minibands and the consequent advantageous effects on the absorption spectrum.